Numerous methods and techniques have been devised in the past to alleviate the problem of unwanted vibrations produced by such sources as vibrating machinery. Although such vibrations cannot usually be completely eliminated by any means, any attenuation of the vibrations is desirable in order to both lessen their destructive effects and increase human comfort.
The earliest attempts to control vibration represent what may be termed "passive" vibration control systems. In such a system, energy damping elements or materials are arranged so as to absorb the unwanted vibrations and dissipate them as heat. Sound insulation and automobile shock-absorbers are two common examples. Other passive vibration control systems include energy-storing elements such as springs which present a reactive load to the vibrating source. For example, large motors are sometimes mounted on springs which have a spring constant chosen in accordance with the frequency at which the motor is known to vibrate. Such springs, by storing and re-emitting the vibrational energy, essentially serve as mechanical filters to lessen the vibrational force transmitted to the surface upon which the motor is mounted.
More recently, improved systems for vibration control have appeared which may be termed "active" vibration control systems. Such systems utilize an actuator for producing vibrations to cancel out the unwanted vibrations, the actuator being under the control of a feedback control system with a vibration sensing means for deriving the error signal used to drive the actuator. Examples of such active systems are found in the U.S. Pat. Nos. 4,566,118 and 4,490,841, the disclosures of which are hereby incorporated by reference.
It is imperative in active vibration control systems that the actuator be capable of producing vibrations in strict accordance with the input signal used to drive the actuator. That is, the preferred relationship between the input signal and the vibrational force produced by the actuator should be a linear one. One type of electromagnetic actuator which is very suitable for vibration control applications is the moving coil type of arrangement commonly used to drive loudspeakers. In such an actuator, an input current is applied to a solenoidal coil which is subjected to a constant magnetic field directed perpendicularly to the direction of the current within the coil. The coil thus experiences a force which is proportional to the input current and directed perpendicularly to the direction of both the magnetic field and current. In the case of loudspeakers, the moving coil (a.k.a. the voice coil) is attached to a diaphragm for producing sound waves in accordance with the input signal. In the case of a motor or actuator, on the other hand, the moving coil is mounted around a slidably mounted shaft which undergoes reciprocating motion in correspondence to the input current. A variation of the moving coil type of actuator involves the placement of one or more stationary coils around a slidably mounted shaft upon which is mounted a permanent magnet. The N-S pole of the magnet is coaxial with the slidably mounted shaft and the surrounding coil. Applying current to the stationary coil causes a magnetic field to be produced which moves the shaft in one direction or the other with a force proportional to the input current applied to the coil.
As aforesaid, for active vibration control applications, an actuator needs to respond in as linear a fashion as possible to its driving input signal. To obtain such a linear response, several design considerations arise. First, the force experienced by the moving element (i.e., the moving coil or moving permanent magnet) should be the same for a given amount of input current irrespective of the moving element's position along its line of travel. This means that ideally the magnetic field produced by the permanent magnet should be uniform all along the stroke of the actuator.
Second, the coil current should follow the input voltage signal as closely as possible. The coil to which the input voltage signal is applied, however, is an inductive load which means that the coil current cannot change instantaneously in response to a change in the input signal voltage. Rather, the coil current responds with a time constant proportional to the inductance of the coil. For linear operation, this time constant should be as small as possible which means the inductance should be minimized.
Third, physical law says that a current-carrying conductor subjected to a constant magnetic field experiences a force proportional to the magnitude of the current. In order to increase the efficiency of the actuator, a suitable flux path should be provided which concentrates the constant magnetic field produced by the permanent magnet and causes it to be directed perpendicularly toward the coil. This maximizes the force experienced by the coil (or magnet in the case of a moving magnet type actuator) for a given amount of coil current and permanent magnet strength.
The current which flows through the actuator coil, however, itself produces a magnetic field which changes the field produced by the permanent magnet. In order to maintain the proportionality between input current and force, the change of the field produced by the magnet should be eliminated, resulting in a force per unit of current which is independent of the amount and direction of the current.
Several prior devices described in the literature represent attempts to partially solve the problems mentioned above. For example, U.S. Pat. No. 4,692,999, issued to Frandsen, describes an electromagnetic actuator of the moving coil variety which uses dual coils and permanent magnets in order to more nearly maintain the uniformity of the magnetic field irrespective of coil position. Japanese Patent Application No. W081/02501 discloses an electromagnetic transducer wherein a compensating coil is mounted coaxially with the moving coil in order to produce a magnetic field in opposition to that produced by the moving coil. U.S. Pat. No. 3,202,886, issued to Kramer, discloses an actuator of the moving permanent magnetic type for two-position operation which makes use of two oppositely energized coils. None of these references, however, teach or suggest an actuator which overcomes the aforementioned problems to the extent necessary for active vibration control applications.